CN102737949B - For the radio-frequency choke transmitted to the gas of RF driven electrode in plasma processing tool - Google Patents

Abstract

In a large area plasma processing system, the process gas is introduced into the chamber through a showerhead assembly, which can be driven as an RF electrode. The grounded gas supply tube is electrically isolated from the spray head. The gas supply tube provides not only process gas, but also cleaning gas from a remote plasma source to the process chamber. The inner side of the gas supply tube can be maintained at a low or zero RF field to avoid early gas decomposition in the gas supply tube which could lead to parasitic plasma formation between the gas source and the jet body. By supplying the gas through the RF choke, the RF field and process gases can be introduced into the process chamber through a common location, thus simplifying the chamber design.

Description

RF for gas delivery to RF-driven electrodes in plasma process equipment Choke

This application is a PCT application "RF CHOKE FOR GAS DELIVERY TO AN RF DRIVERELECTRODE IN A PLASMA PROCES SING APPARATUS" with the original application date of June 25, 2008, the original application number 200880024220.0, and the original international application number PCT/US2008/068148 divisional application.

technical field

Embodiments of the invention generally relate to a radio frequency (RF) choke and gas supply tube for impedance matching in plasma processing equipment.

Background technique

As the demand for larger flat panel displays continues to increase, the size of substrates and process chambers also increases. Higher RF fields are sometimes required when the demands on the solar panel increase. One method of depositing material on a substrate for a flat panel display or solar panel is plasma assisted chemical vapor deposition (PECVD). During PECVD, a process gas is introduced into the process chamber through a showerhead, and the process gas is ignited into a plasma by an RF field, which is applied to the showerhead. As the size of the substrate increases, so does the RF field applied to the jet head. As the RF field increases, the likelihood of premature gas decomposing before the gas passes through the jet increases, and the likelihood of parasitic plasma forming above the jet increases.

Therefore, there is a need in the art for an RF choke and gas supply tube that reduces early gas decomposition and parasitic plasma formation.

Contents of the invention

In a large area plasma processing system, the process gas is introduced into the chamber through a shower head assembly, which can be used as an RF electrode drive. The grounded gas supply tube is electrically isolated from the spray head. The gas supply tube provides not only process gas, but also cleaning gas from a remote plasma source to the process chamber. The inner side of the gas supply tube can be maintained at a low or zero RF field to avoid early gas decomposition in the gas supply tube, which may lead to parasitic plasma formation between the gas source and the jet body. By supplying the gas through the RF choke, the RF field and process gases can be introduced into the process chamber through a common location, thus simplifying the chamber design.

In one embodiment, the RF choke assembly includes a gas supply tube comprising metal and one or more ferrous components surrounding the gas supply tube.

In another embodiment, an apparatus is disclosed. The apparatus includes: a source of RF power; a source of gas; and an RF choke assembly coupled between the source of RF power and the source of gas. The assembly includes a gas supply tube comprising metal. The gas supply tube may include a first end coupled to the gas source and a second end coupled to the RF power source. The gas supply tube also includes one or more ferrous salt components surrounding the gas supply tube.

In another embodiment, a method of gas delivery is disclosed that includes flowing gas through the inside of a metal tube. The metal tube includes a first end coupled to a gas source and a ground, and a second end coupled to an RF power source. The method also includes flowing the RF current along the outside of the metal tube, whereby the gas flowing inside the metal tube is not exposed to the RF current.

Description of drawings

In order to make the above-mentioned features of the present invention more comprehensible, reference may be made to the description of the embodiments, part of which is shown in the accompanying drawings. It should be noted that although the accompanying drawings disclose specific embodiments of the present invention, the accompanying drawings are not intended to limit the spirit and scope of the present invention, and any person skilled in the art may make various modifications and modifications to achieve equivalent implementation example.

FIG. 1A shows a schematic cross-sectional view of a PECVD apparatus according to an embodiment of the present invention.

Figure 1B, schematically enlarged view showing a portion of Figure 1A.

Figure 2A, shows a schematic view of an RF choke and gas supply assembly according to one embodiment of the present invention.

Figure 2B, shows the end view of Figure 2A.

Fig. 3 shows a schematic view of an RF choke and gas supply assembly according to another embodiment of the present invention.

Fig. 4 shows a schematic sectional view of an RF choke according to another embodiment of the present invention.

Fig. 5 shows a schematic cross-sectional view of an RF choke according to another embodiment of the present invention.

Fig. 6 is a schematic cross-sectional view of a gas supply pipe according to an embodiment of the present invention.

Figure 7A shows a schematic cross-sectional view of an RF choke coupled to a plasma process chamber in accordance with one embodiment of the present invention.

Fig. 7B shows the circuit diagram of Fig. 7A.

Figure 8A shows a schematic cross-sectional view of an RF choke coupled to a plasma process chamber according to another embodiment of the present invention.

FIG. 8B shows the circuit diagram of FIG. 8A.

Figure 9A shows a schematic cross-sectional view of an RF choke coupled to a plasma process chamber according to another embodiment of the present invention.

FIG. 9B shows the circuit diagram of FIG. 9A.

Figure 10A shows a schematic cross-sectional view of an RF choke coupled to a plasma process chamber according to another embodiment of the present invention.

Fig. 10B shows the circuit diagram of Fig. 10A.

FIG. 11A shows a schematic view of an RF choke 1100 according to another embodiment of the present invention.

FIG. 11B shows a schematic cross-sectional view of the RF choke 1100 of FIG. 11A.

For ease of understanding, the same component symbols in the drawings represent the same components. Components employed in one embodiment can be applied to other embodiments without specific elaboration.

detailed description

In a large area plasma processing system, process gas may be introduced into the chamber through a showerhead assembly, and the showerhead assembly is driven as an RF electrode. The gas supply tube is grounded, and the gas supply tube is electrically isolated from the gas shower head. The gas supply tube can provide not only the process gas, but also the cleaning gas from the remote plasma source to the process chamber. The inner side of the gas supply tube can be maintained at a low or zero RF field to avoid early gas decomposition in the gas supply tube, resulting in the formation of a parasitic plasma between the gas source and the gas shower head. By routing the gas supply through the RF choke, the RF field and process gases can be introduced into the process chamber through a common location, thus simplifying the process chamber design.

The invention will be described below with reference to a PECVD chamber available from AKT, a subsidiary of Applied Materials, Santa Clara, California. It should be understood that the invention is equally applicable to any chamber requiring the use of RF current to excite a gas into a plasma, including physical vapor deposition (PVD) chambers. It should also be understood that the invention described below is equally applicable to PECVD chambers, etch chambers, physical vapor deposition chambers, plasma processing chambers, and other chambers made by other manufacturers.

FIG. 1A shows a schematic cross-sectional view of a PECVD apparatus 100 according to one embodiment of the present invention. Apparatus 100 includes a lid assembly 102 coupled to a chamber wall 108 . Within the apparatus 100, a gas shower head 110 is positioned opposite a susceptor 104 on which a substrate 106 is positioned for processing. The spray head 110 may be supported by a bracket 140 . The substrate 106 can enter and exit the apparatus 100 through a slit valve 122 disposed in the chamber wall 108 . The substrate 106 includes a flat panel display substrate, a solar substrate, a semiconductor substrate, an organic light emitting display (OLED) substrate, or any other substrate. The gas shower head 110 may include one or more gas channels 112 extending between the top surface 118 and the bottom surface 120 of the gas shower head 110 . The gas filling part 114 may exist between the upper cover assembly 102 and the air spray head 110 . The gas introduced into the gas filling part 114 can be evenly distributed behind the gas shower head 110 to be introduced into the process space 116 through the gas channel 112 .

Gas is introduced into the plenum 114 via a gas input 138 . The gas may be provided by a gas source 126 . In one embodiment, the gas source 126 may include a process gas source. In another embodiment, the gas source 126 may include a clean gas source. Gas may be diverted from gas source 126 to RF choke 132 through remote plasma source 128 and cooling coupling 130 . RF choke 132 may be coupled to divert connection 136 that supplies gas into gas input 138 . The RF power source 124 may also be coupled to a steering connection 136 through an RF supply 134 .

On the face of it, it is not safe to couple gas and RF power through a common location. However, RF current can have a skin effect when passing through a conductive surface. The RF current moves as close as possible to the source driving itself. Therefore, the RF current moves over the surface of the conductive component, and the RF current penetrates only a certain predetermined depth (ie, skin) of the conductive component. The predetermined depth can be calculated as a function of the RF current frequency, the permeability of the material of the conductive component, and the conductivity of the conductive material. Therefore, when the conductive component is thicker than the predetermined depth penetrated by the RF current, the RF current does not directly react with the gas flowing in the conductive component. FIG. 1B is a schematic cross-sectional view of a portion of FIG. 1A . Figure 1B shows the path that the RF current travels to the showerhead (indicated by arrow A), and through the RF choke (indicated by arrow B).

As shown in FIG. 1B , the RF current flows on the surface of the RF supply, the outside of the gas input 138, the top of the lid assembly 102, the outside edge of the lid assembly 102, the surface of the bracket 140 relative to the plenum 114, and Ultimately over the bottom surface 120 of the gas shower head 110 . Once the RF current reaches the bottom surface 120 of the showerhead 110 , the gas can be ignited into a plasma in the empty cathode chamber, which exists in the gas channel 112 , or in the process volume 116 . The RF current may also travel along the top of the steering connection 136 and the outside of the RF choke 132 . The longer the path that the RF current travels, the greater the impedance. Thus, for centrally supplied RF current, as the size of the chamber becomes larger, the impedance of the airjet head increases.

An RF choke may be used between the RF power source and the gas source to ensure that the voltage difference between the RF power source and the gas delivery system exhibits a substantially uniform attenuation. The voltage drop across the RF choke may be approximately equal to the voltage level of the gas distribution jet. Additionally, the voltage drop along the RF choke may be substantially uniform. Thus, the RF power output from the RF power source used to maintain and ignite the plasma within the process chamber is then known and repeatable. The RF choke can maximize the voltage delivered to the jet, and thus make the impedance of the RF source substantially equal to the impedance of the load.

FIG. 2A is a schematic view of an RF choke 200 according to one embodiment of the invention. FIG. 2B is a cross-sectional view of FIG. 2A. The RF choke 200 includes a coil 202 wound around a ferrous material 204 . A cooling pipe 206 is provided inside the ferrous salt material 204 . In one embodiment, cooling tube 206 includes copper. One or more fins 208 extend radially from the cooling tube 206 to further extend the cooling surface outward into the ferrous material 204 and dissipate heat from the coil 202 . Coil 202 may be wound around ferrous material 204 multiple times. Coil 202 may be thick enough to prevent RF current from penetrating inside coil 202 . One end 210 of the coil 202 is coupled to a gas input to connect to the process chamber, and the other end 212 is coupled to ground and a gas source. Thus, RF current flows from the gas input along the outside or surface of the coil 202 to ground.

As the RF current moves, the inductance increases. After a sufficient distance, the inductance along the RF choke can be substantially equal to the inductance of the jet. The inductance can be increased by increasing the radius, length or number of turns of the coil 202 . Additionally, as the RF current travels along the outside of the coil 202, the RF current contacts the ferrous material 204 and causes a high impedance. The length of the coil 202 should be short enough to ensure that the impedance of the RF choke is not greater than the impedance of the load of the jet head. If the impedance of the RF choke is greater than the impedance of the load, the RF choke 200 will fail.

The ferrous salt material 204 may include a high-frequency low-loss ferrous salt material. In an embodiment, the ferrous salt material 204 may comprise semi-cylindrical blocks that form a dense cylindrical ferrous salt core. In another embodiment, the ferrous material 204 may comprise a quarter-cylindrical block forming a dense cylindrical ferrous core. The ferrous salt material 204 increases the permeability and thus the inductance. Due to the capacitance created by the RF choke 200 , the resonance of the RF current is reduced by the ferrous material 204 coupled with the increased RF path provided by the coil 202 . The RF choke 200 has high RF impedance to create RF-to-ground isolation.

Ferrous material 204 increases permeability and also increases inductance. The ferrous material 204 additionally provides additional voltage drop between the RF source and ground. The ferrous salt material 204 can act as a thermal insulator, thereby reducing heat loss from the coil 202 .

The coil 202 may comprise aluminum, and the coil 202 is sufficiently thick to prevent RF current from seeping into the inside of the coil (ie, where the gas flows). In one embodiment, the coil 202 may comprise hard anodized aluminum. In another embodiment, the coil 202 comprises stainless steel. The inner surface of the coil 202 is resistant to cleaning gases such as fluorine and fluorine radicals from a remote plasma source (RPS). Coil 202 may have a large cross-sectional area to allow high gas conductance, thus allowing a safe pressure window for stable RPS operation. Since the RF field does not penetrate the inside of the coil 202, the gas passing through the coil 202 does not encounter the RF field, and thus the gas does not ignite into a plasma. In other words, the inner side of the coil 202 may include a field free region. Compared to the RF current entering the jet, any current of the present invention that penetrates the coil 202 or does not diverge from the ends of the coil and thus encounters the gas is very slow and thus does not form a plasma. If a plasma is formed in the RF choke 200, the amount of RF current flowing to the RF choke 200 will increase, thereby causing a decrease in the RF current flowing to the showerhead. In one embodiment, no ferrous salt component may be provided.

FIG. 3 is a schematic cross-sectional view of an RF choke 300 according to another embodiment of the present invention. The RF choke 300 includes a gas tube 302 having one or more cooling fins 304 extending radially outward from the gas tube 302 . In one embodiment, heat sink 304 includes aluminum. One or more ferrous salt pans 306 may surround gas tube 302 and be disposed between fins 304 . In one embodiment, the ferrous salt disk 306 may comprise a low-loss ferrous salt having half-toroidal pairs coupled together to form an electromagnetically continuous toroidal coil ( toroid). In another embodiment, the ferrous salt pans 306 may comprise low-loss ferrous salts that are ring-shaped and each completely surround the gas tube 302 . It should be understood that other shapes of ferrous salt pan 306 may also be used. A first end 308 of the RF choke 300 may be coupled to a gas input of the process chamber, and a second end 310 of the RF choke 300 may be coupled to ground. The RF current can travel along the RF path C on the outside of the gas tube 302 and along the heat sink 304 . In one embodiment, the cooling fins 304 may radially extend a distance from the gas pipe 302 , and the distance is greater than the radial distance of the ferrous plate 306 from the gas pipe 302 . To accommodate high RF currents, the gas tube 302 can be lengthened, and more ferrous salt pans 306 and heat sinks 304 can be added. In one embodiment, the distance that the heat sink 304 extends from the gas tube 302 may be increased. In one embodiment, the RF choke 300 may be cooled by drilling cooling channels in the gas tube 302 . In one embodiment, the ferrous salt plate 306 may not be provided.

FIG. 4 is a schematic cross-sectional view of an RF choke 400 according to another embodiment of the present invention. The RF choke 400 includes gas tubes 402 separated from each other by one or more O-rings 404 extending radially outward from the gas tubes 402 . In one embodiment, the O-ring 404 may comprise silicone rubber. One or more O-rings 404 may allow air to circulate around the ferrous plates 406 and moderate the impact of the ferrous plates 406 rubbing against each other. The O-ring 404 can separate adjacent ferrous salt discs 406 by a predetermined distance. One or more ferrous plates 406 may also surround gas tube 402 and be disposed between O-rings 404 . In one embodiment, the O-ring 404 may be replaced with a spacer element that creates a small distance between adjacent ferrous plates 406 . In one embodiment, an air gap is formed between adjacent ferrous salt trays 406 . In an embodiment, the ferrous salt plate 406 may comprise a single material that surrounds the gas tube 402 and spans a predetermined length of the gas tube 402 .

In one embodiment, the ferrous salt disk 406 includes a low loss ferrous salt having half-toroidal pairs coupled together to form an electromagnetically continuous toroidal coil. In another embodiment, the ferrous salt pans 406 may comprise low-loss ferrous salts that are ring-shaped and each completely surround the gas tube 402 . A first end 408 of the RF choke 400 may be coupled to a gas input of the process chamber, while a second end 410 of the RF choke 400 may be coupled to ground. The RF current may travel along the RF path D on the outside of the gas tube 402 . To accommodate high RF currents, the gas tube 402 can be lengthened and more ferrous salt pans 406 added. In an embodiment, the RF choke 400 may be cooled by drilling cooling channels in the gas tube 402 . In one embodiment, the ferrous salt pan 406 may not be provided.

FIG. 5 is a schematic cross-sectional view of an RF choke 500 according to another embodiment of the present invention. The RF choke 500 includes a gas supply tube 502 through which process gas can flow. Cylindrical portion 504 may surround gas tube 502 . In an embodiment, the cylindrical portion 504 may include a conductive material. In another embodiment, cylindrical portion 504 may comprise metal. Cylindrical portion 504 may include a first end 510 and a second end 512 . The first end 510 may have a substantially closed end, and the second end 512 may include a substantially open end and have one or more extensions 514 extending from the first end 510 . Between the extensions 514, a ferrous material 506 may be disposed. Additionally and/or optionally, an air pocket 508 is disposed between the extensions 514 . The RF current travels along the outside of gas tube 502 (arrow E) and along the surface of cylinder 510 including extension 514 . As such, impedance increases due to the increased RF path length and exposure of the ferrous material 506 .

FIG. 6 shows a schematic cross-sectional view of a gas supply pipe 600 according to an embodiment of the present invention. The gas supply tube 600 includes an inner tube 602 substantially surrounded by an outer tube 604 . The inner tube 602 is separated from the outer tube 604 by an outer passage 608 . Cooling fluid may move through the outer channel 608 as indicated by arrow G . To make the cooling fluid follow a more non-linear path, thus increasing the residence time in the outer channels, the cooling fluid may encounter several turbulences 606 to change its flow path. In one embodiment, the disruptor 606 may be a metal wire that is wound around the inner tube 602 . In another embodiment, the disruptor 606 may include one or more flanges extending between the inner tube 602 and the outer tube 604 . Process gases may flow along the inner passage 610 in the inner tube 602 as indicated by arrows F .

7A is a schematic cross-sectional view of an RF choke coupled to a plasma process chamber in accordance with one embodiment of the present invention. FIG. 7B is a circuit diagram of FIG. 7A. The process gas enters the system 700 through the RF choke 706 and the back plate 702 . RF power is supplied by RF power source 704 . The RF voltage at the gas inlet (ie, the ground side of the RF choke 706) is zero. An RF choke 706 is connected in parallel with the load. The RF choke 706 is designed to have high RF impedance and can be inductive or capacitive. The RF choke 706 may operate at or near resonance with or without the assistance of external and/or stray capacitance. In terms of an equivalent electrical circuit, the Pi network 708 may include a load capacitor C _L , a tuning capacitor C _T , and an impedance Z _FT of the RF choke. The load impedance Z _L may include the impedance Z _FT of the RF choke and the impedance Z _CH of the chamber.

8A is a schematic cross-sectional view of an RF choke coupled to a plasma process chamber in accordance with one embodiment of the present invention. FIG. 8B is a circuit diagram of FIG. 8A. The process gas enters the system 800 through the RF choke 806 and the back plate 802 . RF power is supplied by RF power source 804 . The RF voltage at the gas inlet (ie, the ground side of the RF choke 806) is zero. An RF choke 806 is connected in parallel with the load. The RF choke 806 is designed capacitively with or without an external capacitive load _{CL '} , _{CL "} which forms a load capacitor in the inverse L-shaped matching network 808 C _L . In terms of an equivalent circuit, the reverse L-shaped matching network 808 may include a load capacitor _CL , a tuning capacitor C _T , and an impedance Z _FT of the RF choke. The load capacitor _CL may include the impedance Z _FT of the RF choke and the external capacitive load _CL' . The RF choke 806 can be considered as part of the load capacitor in the inverting L-shaped matching network 808 .

9A is a schematic cross-sectional view of an RF choke coupled to a plasma process chamber in accordance with one embodiment of the present invention. FIG. 9B is a circuit diagram of FIG. 9A. The process gas enters the system 900 through the RF choke 906 and the back plate 902 . RF power is supplied by RF power source 904 . The RF choke 906 may be considered as part of a tuning capacitor _CT in an L-shaped or Pi matching network 908 . RF choke 906 is in series with the load. The RF choke 906 is designed capacitively with or without an external tuning capacitor C _{T '} which forms the tuning capacitor in the L-shaped matching network. In terms of an equivalent circuit, the L-shaped matching network may include a load capacitor _CL and a tuning capacitor C _T . The tuning capacitor C _T may include the impedance Z _FT of the RF choke as well as the external tuning capacitor C _T ′.

10A is a schematic cross-sectional view of an RF choke coupled to a plasma process chamber in accordance with one embodiment of the present invention. FIG. 10B is a circuit diagram of FIG. 10A. The process gas enters the system 1000 through the RF choke 1006 and the back plate 1002 . RF power is supplied by RF power source 1004 . The RF voltage at the gas inlet (ie, the ground side of the RF choke 1006) is zero. Two RF chokes 1006 or two sections of one RF choke 1006 are put together to become the loading and tuning components in L-type or Pi-type matching network 1008 . The RF choke 1006 is designed capacitively, with or without external load and tuning capacitors _CL' and _CT' . In terms of equivalent circuits, the network may include a load capacitor _CL and a tuning capacitor _CT . The load capacitor _CL may include an external load capacitor _CL' and an impedance Z _FT1 of the first RF choke. The tuning capacitors may include the impedance Z _FT2 of the second RF choke and the external tuning capacitor C _T′ .

11A is a schematic view of an RF choke 1100 according to another embodiment of the present invention. FIG. 11B is a schematic cross-sectional view of the RF choke 1100 of FIG. 11A taken along the section line H-H. As shown in FIGS. 11A and 11B , the ferrous salt component 1104 extends lengthwise along the gas tube 1102 . One or more ferrous salt assemblies 1104 may be disposed on and substantially cover the outer surface of the gas tube 1102 .

By placing a choke between the gas source and the process chamber, parasitic plasma can be reduced. The RF choke may comprise a gas tube with a wall thickness greater than the maximum desired penetration thickness of the RF current. Additionally, the RF choke may have a sufficiently long RF path such that the impedance of the RF choke is substantially equal to the impedance of the load to the showerhead.

Although the present invention has been described as above with preferred embodiments, it is not intended to limit the present invention. Any changes and modifications made by those skilled in the art without departing from the spirit and scope of the present invention should still belong to the technology of the present invention. category.

Claims (12)

1. An RF choke assembly comprising: a gas supply tube comprising metal; and a plurality of ferrous salt assemblies directly coupled to and at least partially surrounding the gas supply pipe, wherein the plurality of ferrous salt assemblies span the gas supply The predetermined length of the tube. 2. The RF choke assembly of claim 1, wherein the one or more ferrous assemblies comprise a plurality of ferrous plates. 3. The RF choke assembly of claim 1, wherein the gas supply tube comprises aluminum. 4. An RF choke assembly comprising: a gas supply tube comprising metal; and a plurality of ferrous salt assemblies directly coupled to and at least partially surrounding the gas supply pipe, wherein the plurality of ferrous salt assemblies span the gas supply A predetermined length of tubing wherein the one or more ferrous salt assemblies include a plurality of ferrous salt plates, wherein adjacent ferrous salt plates are separated by one or more O-rings. 5. The RF choke assembly of claim 4, wherein the gas supply tube comprises aluminum. 6. A plasma process equipment comprising: RF power source; gas source; and an RF choke assembly coupled between the RF power source and the gas source, the assembly comprising: a gas supply tube comprising metal, a first end coupled to the gas source, and a second end coupled to the RF power source; and a plurality of ferrous salt assemblies directly coupled to and at least partially surrounding the gas supply pipe, wherein the plurality of ferrous salt assemblies span the gas supply The predetermined length of the tube. 7. The apparatus of claim 6, wherein the second end of the gas supply tube is grounded. 8. The apparatus of claim 7, wherein the one or more ferrous salt assemblies comprise a plurality of ferrous salt pans. 9. The apparatus of claim 6, wherein the gas supply tube comprises aluminum. 10. A plasma processing apparatus comprising: RF power source; gas source; and an RF choke assembly coupled between the RF power source and the gas source, the assembly comprising: a gas supply tube comprising metal, a first end coupled to the gas source, and a second end coupled to the RF power source; and a plurality of ferrous salt assemblies directly coupled to and at least partially surrounding the gas supply pipe, wherein the plurality of ferrous salt assemblies span the gas supply a predetermined length of pipe, wherein said second end of said gas supply pipe is grounded, wherein one or more ferrous salt assemblies comprise a plurality of ferrous salt pans, wherein adjacent ferrous salt pans are covered by one or more O separated by rings. 11. The apparatus of claim 10, wherein the gas supply tube comprises aluminum. 12. A method of gas delivery comprising: flowing gas to the process chamber through the inside of a metal tube, the metal tube including a first end coupled to a gas source and to ground, and a second end coupled to an RF power source; and When the gas flows through the metal tube, the RF current is caused to flow along the outside of the metal tube, whereby the gas flowing inside the metal tube, where the metal The tube has a plurality of ferrous components directly coupled to the metal tube and at least partially surrounding the metal tube, wherein the plurality of ferrous components span the metal tube predetermined length.